Prevention of corrosion of ferrous metals by alkanolamines



United States Patent Oflfice 2,715,605 Patented Aug. 16, 1955 PREVENTIONOF C(BRRGSION OF FERROUS METALS BY ALKAYOLAMINES Joseph K. Goerner,Austin, Tera, assignor to Jeiierson Chemical Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Appiication July 2, 1952,Serial No. 295,961

9 Claims. (Cl. 202-57) This invention relates to the prevention ofcorrosion of ferrous metals by alkanolamines and to alkanolaminecompositions which are substantially non-corrosive to ferrous metals.More particularly, this invention relates to minimizing the corrosion offerrous equipment in which alkanolamines are distilled or stored orwhich equipment is employed for carrying out processes usingalkanolamines. By ferrous metals is meant structural steel includingsteels containing chromium, and/ or nickel, stainless steel, cast iron,Wrought iron and other forms of steel and iron employed in theproduction of tanks, stills, pipe lines and other equipment forhandling, storing or transporting alkanolamines.

Alkanolamines, such as monoethanolamine, diethanolamine,triethanolamine, isopropanolamine, diisopropanolamine,triisopropanolamine, etc., are commonly prepared by allowing aqueousammonia solutions to react with ethylene oxide, propylene oxide, etc.N-dimethyl ethanolarnine may be made by allowing ethylene oxide to reactwith aqueous dimethylamine, and N-methyl monoethanol amine and N-methyldiethanolamine are prepared by reaction of ethylene oxide with aqueousmethyl amine. By employing various different amines and alkylene oxidesa great variety of different alkanolamines may be obtained; preferredalkanolamines contain from 2 to 18 carbon atoms.

The reaction of an alkylene oxide with ammonia inevitably results in amixture of alkanolamines. Thus, for example, the reaction of ethyleneoxide with ammonia yields a mixture of mono-, di-, andtri-ethanolamines. This mixture must be resolved to obtain commerciallyacceptable products. This is usually accomplished by distilling thecrude alkanolamines, preferably under vacuum. When carrying out thisdistillation in ferrous equipment considerable iron dissolves in the hotliquid alkanolamines. Such corrosion of ferrous equipment is aggravatedby the presence of moisture or elemental oxygen in the alkanolaminesolution.

In carrying out certain porcedures involving the use of alkanolamines,for example, the scrubbing of gases such as petroleum refinery gases,coke oven gas and other combustible gases in ferrous equipment to removecarbon dioxide and hydrogen sulfide from the gases, considerablecorrosion of the equipment takes place, particularly during theregeneration of the scrubbing medium by heating it usually attemperatures above C., usually above to C., to expel the dissolved gas.At these temperatures, particularly at about C., the corrosion isappreciable, and at C. and higher presents a serious problem. In anendeavor to minimize such corrosion in the case of gas purificationprocesses involving the use of ethanolamines as the scrubbing medium ithas been proposed to add to the scrubbing solution of ethanolamines asmall amount of arsenic compounds, such as arsenous oxide and sodiumarsenite, vanadium oxide, copper salts or iodine compounds, as corrosioninhibitors. Of these corrosion inhibitors the arsenic compounds appearto be most eifective to inhibit corrosion during distillation ofalkanolarnines. The use of arsenous oxide or sodium arsenite ascorrosion inhibitors in such process, however, has the seriousdisadvantage that some of the arsenic compound volatilizes with thealkanolamines and appears in the distillate contaminating the same.

It is among the objects of this invention to provide a process forminimizing the corrosion of ferrous metals by alkanolamines, whichprocess is simple to carry out and does not result in the contaminationof the alkanolamine distillate when the process is employed to minimizecorrosion of ferrous equipment in which alkanolamines are distilled.

Another object is to provide an alkanolamine suitable for scrubbinggases to remove carbon dioxide and/or hydrogen sulfide and for otheruses and which is substantially non-corrosive to ferrous metals.

Other objects and advantages of this invention Will be apparent from thefollowing detailed description thereof.

This invention is based upon the surprising discovery that the corrosionof ferrous equipment in contact with hot alkanolamines at a temperatureabove 150 C. can be greatly minimized, if not substantially completelyprevented, by the addition of a small amount of an antimony compound tothe alkanolamines. By antimony compound is intended to include antimonyas such, as well as antimony in combined form. Examples of antimonycompounds which can be used are elemental antimony, antimony trioxide(SbzOs), antimony pentaoxide (SbzOs), antimony tetraoxide (Sb2O4),alkali salts including the alkali metal and alkaline earth metal salts,e. g., sodium, potassium, lithium, calcium, magnesium salts ofantimonous and antimonic acids, potassium antimonyl tartrate, known astartar emetic, other alkali metal and alkaline earth metal antimonyltartrates, such as sodium antimonyl tartrate and other organic antimonycontaining compounds such as the antimony acetates. The preferredcorrosion inhibitor is antimony trioxide.

When adding the antimony inhibitor to alkanolamines subjected todistillation in ferrous equipment, it is preferred to employ aninorganic antimony compound to avoid adding to the distillation systemorganic material which decomposes at the high temperatures employed inthe distillation and thus tends to contaminate the distillate andpossibly result in a product having poor color stability.

The amount of antimony compound added to the alkanolamine may be from0.001% to 5.0% by Weight based on the weight of the alkanolamines,preferably from 0.01% to 1% by Weight. The antimony compound may beadded continuously or intermittently to the alkanolamine solution toinsure the presence of an amount of antimony inhibitor within the rangeabove noted. In the case of the distillation of alkanolamines it ispreferred to add the antimony inhibitor continuously, for example, bypreparing a slurry of antimony trioxide in say diethanolamine containingabout 10% by weight of the antimony trioxide and feeding this slurryinto the distillation system at any point which permits the antimonystream to join the crude alkanolamine stream before the latter is heatedto about 150 to 160 C. Thus, in the case of the distillation ofethanolamines the antimony containing slurry may be added either to thecrude aqueous reaction mixture which Will contain Water, ammonia, mono-,diand tri-ethanolamines as well as high boiling residues, or it may beadded to a relatively anhydrous ethanolamine mixture before the latteris subjected to distillation to resolve it into its components.

While the explanation for the behavior of antimony compounds to minimizecorrosion of ferrous equipment by alkanolamines is not fully understood,it is believed that the addition of the antimony compound to the 3alkanolamines coats or plates with a thin film of antimony the ferrousmetal surfaces with which the alkanolamines would otherwise come incontact. As long as this antimony coating is present the underlyingferrous metal does not corrode to a serious extent. If no antimonycompound is added after the formation of the initial coating or plating,the protection lasts for a short time, say from a few minutes to severalhours,'depending on such factors as the relative amount of antimonycompound added to the system, the surface to volume ratio of the system,temperature, rates of flow, amount of oxygen or moisture present, etc.After this short period of time, recominencement of corrosion takesplace and is indicated by the appearance of excessive amounts of iron inthe liquid being discharged from the still or other vessel in which 'thehot alkanolamines are treated or stored. Further Example I A sample ofmild steel with an area of cm. was immersed in 1000 ml. of a liquidmixture containing approximately monoethanolamine, 40% amine, 19%triethanolamine and 1% water disposed in a 2000 ml. flask, to which wasattached a reflux condenser. The liquid was protected from theatmosphere by a stream of dry nitrogen gas introduced into the flaskbelow the liquid level. After 51 hours at 190 C. it was 7 r found thatthe steel sample showed an average penetration rate of 0.09 inch peryear, and that the ethanolamines liquid had picked up 355 parts of ironper million parts of ethanolamines.

' Ina comparative test carried out in exactly identical manner, exceptthat 0.027%' of SbzOs by weightbased on the weight of the ethanolamineswas added to the ethanolamines before the start of the heating, thepeneitration was 0.0019 inch per year, and the iron pick-up wasonly 29parts per million parts of ethanolamines.

Example II .A commercial unit of mild steel for thecontinuous'distillation of monoethanolamine was operated for 48 hourswithout any corrosion inhibitor. It was found that the still bottomsliquid contained 600 parts of iron per million parts ofmonoethanolamine. Addition of antimony trioxideto the feed was thencommenced at a rate of 0.03% by weight based on the weight of theethanolamaine fed to the still. After a very short time the iron contentof the still bottoms liquid fell to 50 parts per million parts ofmonoethanolamine.

Example III This example differs from Example I in that 0.031% by weightof sodium antimonate (NaSbOs) was added to the ethanolamine mixturebefore the start of the heating. The penetration was 0.0060 inch peryear, and the iron pick-up was parts per million parts of ethanolamines.

Example IV A sample of stainless steel withan area of 20 squarecentimeters was immersed in 1000 ml. of a mixture of alkanolamines thesame as that used in Example I. After 51 hours of heating at 185 190 C.it was found penetration was at the rate of 0.0026 inch per year and the'iron pick-up was 44 parts per million parts of alkanolamines.

In a comparative test in which 0.05% by weight of diethanolrosion offerrous metals by hot alkanolamines.

antimony trioxide was added to the amines as inhibitor on the samestainless steel it was found that penetration was at the rate of 0.0008inch per year on the basis of 65.75 hours heating at 185 to 190 C. Theiron pick-up was 12 parts per million parts of alkanolamines.

Example V Diethanolamine containing about 0.05 by weight of antimonytrioxide based on the weight of the diethanolamine and diluted withwater to a concentration of about 8% by weight of diethanolamine isemployed as the scrubbing medium to remove carbon dioxide from boilerstack gas passing through a steel scrubbing tower with no appreciablecorrosion of the steel Walls taking place. The exhausted scrubber liquidis regenerated by heating to about C. in a steel vessel without anyserious corrosion of the steel walls of the regenerator.

It Will be noted this invention effectively inhibits cor- Furthermore,the use of this invention in inhibiting corrosion of ferrous metals,such as stills employed in the distillation of alkanolamines, results inthe production of an alkanolamine product which isnot contaminated bythe corrosion inhibitor.

It will be further noted this invention provides a new composition ofmatter, namely, a mixture of alkanolamines with an antimony compound,which when used as a solvent for removal of carbon dioxide, hydrogensulfide, or for other uses is substantially non-corrosive to ferrousequipment. The preferred compositions are the ethanolamines, mono-, di-,or trior mixtures thereof containing from 0.001% to 5.0% by weight,preferably from 0.01% to 1.0% by Weight, of an antimony compound,preferably an antimony oxide, especially antimony trioxide.

It is to be understood that this invention is not restricted to thepresent disclosure otherwise than as defined by the appended claims.

What is claimed is:

1. A process of minimizing the corrosion of ferrous metals by hot loweralkanolamines, which comprises admixing with said lower alkanolaminesfrom 0.001% to 5.0% by weight of an antimony compoundbased on the weightof the alkanolamines.

2. A process of minimizing the corrosion of ferrous metals by loweralkanolamines at a temperature above C., which comprises maintaining insaid lower alkanolamines a concentration of an antimony compound of from0.001% to 5.0% by weight based on the weight of the alkanolamines.

3. A process of minimizing corrosion of ferrous metals by loweralkanolamines during the distillation of said alkanolamines, whichcomprises admixing with said lower alkanolamines prior to beingsubjected to distillation from 0.001% to 5.0% by weight of an inorganicantimony compound based on,the weight of the alkanolamines.

4. The process defined in claim 3, in which from 0.01 to 1% by Weight ofantimony compound is added to the lower alkanolamines based on theweight of the alkanolamines subjected to distillation.

5. A process of minimizing the corrosion of a steel still in which amixture of lower alkanolamines is subjected to distillation, whichcomprises adding to the stream of said lower alkanolamines introducedinto the still from 0.01%

' to 1% of an antimony oxide by weight based on the weight of thealkanolamines.

6. In a process of resolving by distillation in a ferrous metal vessel amixture of monoethanolamine, diethanolamine and triethanolamine, theimprovement which comprises adding to said mixture from 0.01% to 1% byweight of antimony oxide based on the weight of said mixture ofethanolamines to inhibit corrosion of the ferrous metal walls of thevessel by said ethanolamines.

7. As a new composition of matter a lower alkanolamine containing from0.001% to 5.0% by weight of an 6 antimony compound based on the weightof said lower 0.01% to 1.0% by weight of antimony trioxide based onalkanolamine. the weight of the diethanolamine.

8. As a new composition of matter an ethanolamine containing from 0.001%to 5.0% by weight of an anti- References Clted m the file of thls Patentmony oxide based on the weight of the ethanolamine. 5 UNITED STATESPATENTS 9. As a new composition of matter a solvent consist- 2,031,632Bottoms Feb. 25, 1936 ing primarily of diethanolamine and containingfrom 2,303,399 SChWartz Dec. 1, 1942

5. A PROCESS OF MINIMIZING THE CORROSION OF A STEEL STILL IN WHICH AMIXTURE OF LOWER ALKANOLAMINES IS SUBJECTED TO DISTILLATION, WHICHCOMPRISES ADDING TO THE STREAM OF SAID LOWER ALKANOLAMINES INTRODUCEDINTO THE STILL FROM 0.01% TO 1% OF AN ANTIMONY OXIDE BY WEIGHT BASED ONTHE WEIGHT OF THE ALKANOLAMINES.